Development Evolution 9. Phenotypic plasticity 1. development 2. evolution The short version: 3. epigenetics 4. canalization 5. cryptic genetic variation / capacitance phenotype depends on the environment 6. genetic assimilation 7. evolvability 8. preadaptation 9. phenotypic plasticity 10. epistasis 11. modularity 12. developmental bias / constraint
Phenotypic plasticity: long version Phenotypic plasticity
• Gradual response to environmental gradient OR “When I use a word, it means just what I discrete switching between types choose it to mean – neither more nor less” • Fixed traits (eg size after metamorphosis) OR Humpty Dumpty labile traits (eg behavioral) • Change in the phenotypic mean (reaction norm) OR change in the phenotypic variance “When I make a word do a lot of work like that, • Response to a directional environmental shift OR I always pay it extra” response to residual environmental “noise” Humpty Dumpty • Adaptive, evolved response OR side effect of basic physiology
How does phenotypic plasticity trait Spectrum adaptive tie in to other concepts? phenotypic plasticity Some contradictory (?) ideas: random preadapted • Genetic assimilation of plastic trait is loss not gain mutation cryptic variation of complexity, so genetic assimilation is irrelevant • Random variation unlikely to be adaptive to the evolution of novelty (Williams) • Very deleterious stuff weeded out • Canalization is the opposite of phenotypic • Revealed variation more likely to be adaptive plasticity (Goggy) • Trait needed briefly, then lost • Revelation of cryptic genetic variation in response • Easier to find it again in cryptic variation to stress is one kind of phenotypic plasticity • Frequent environmental switching ⇒ evolution of adaptive phenotypic plasticity (or reaction norm) • Environment stabilizes ⇒ genetic assimilation with loss of complexity
1 10. Epistasis 11. Modularity • General definition: the effect of an allele at one • Pleiotropic effects locus depends on alleles at other loci within but not between • Population genetics definition: departure from modules multiplicative effects, where fitness is the • Evolvability: one module product of independent fitness components can then evolve without • Quantitative genetics definition: departure from disturbing the others additive effects, seen as interaction terms under • Modularity can arise by a linear regression model either parcellation or • Nonepistatic relationships under one definition integration show up as epistatic under the other Wagner & Alternberg 1996
Sting rays descended from sharks 12. Developmental bias / constraint Sharks were • Evolution can only work with variants that appear already a bit flattened • Developmental constraints can stop useful things Rays and evolving skates lie on their bellies • Look at fish living at sea bottom as an example of a developmental constraint that wasn’t quite strong enough
Ancestor of flounders had flattish sides Flounders, plaice and sole lie on their side During development, eyes must twist around to get to same side of body
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